Manufacture of terephthalic acid/2-methylpentamethylene diamine/hexamethylene diamine copolyamides

ABSTRACT

A process for the copolymerization of (1) a carboxylic acid consisting of terephthalic acid and mixtures thereof with isophthalic acid with (2) a mixture of hexamethylene diamine and 2-methyl pentamethylene diamine. The combined amount of isophthalic acid and 2-methyl pentamethylene diamine is controlled so that the resultant copolyamide, when annealed, has less than 1% by weight based on the total weight of the copolyamide of fractions having melting points greater than 320° C. Preferably, the combined amount of isophthalic acid and 2-methyl pentamethylene diamine is greater than 27.5%, molar basis, of the total amount of acid and diamine.

The present invention relates to a process for the manufacture ofcopolyamides of terephthalic acid, and especially to the manufacture ofsuch copolyamides without formation of a high melting fraction.

As used herein, a "single step" polymerization process is apolymerization process in which monomers are subjected to apolymerization cycle in a reactor that concludes with the pressure beingreturned to substantially atmospheric pressure. It is understood thatthe copolyamide obtained from such a process may be subjected to furtherprocessing, including finishing and extrusion, before or afterpelletization.

Polymers formed from terephthalic acid and diamines are known in theart. For instance, PCT patent application CA91/00442 of S. L. Mok and R.U. Pagilagan, filed 1991 Dec. 11, discloses copolyamides of terephthalicacid with hexamethylene diamine and 2-methyl pentamethylene diamine,optionally containing isophthalic acid, having melting points of atleast 280° C. Other polyamides of terephthalic acid are referred totherein.

Single-step polymerization processes have the advantage that polymer ismanufactured in a single step, as opposed to two step or multi-stepprocesses, thereby reducing handling of the partially formed polymerduring the intermediate stages of the process. However, in themanufacture of copolyamides of terephthalic acid, especiallycopolyamides of terephthalic acid, hexamethylene diamine and 2-methylpentamethylene diamine, using a single step polymerization process, ithas been found that a high melting fraction is formed.

The high melting fraction, with a melting point in excess of 320° C. andespecially approximately 334° C., is usually not noticed in the firstbatch of polymer produced in an autoclave, but becomes apparent insubsequent batches made in the same autoclave without steps being takento clean-out the autoclave between consecutive batches. As the polymerwould normally be subsequently processed at melt temperatures of lessthan 320° C., the high melting fraction manifests itself as unmelted orgel-like portions of polymer, which are commercially unacceptable; thegel-like portions are believed to be essentially comprised of a highmelting crystalline phase, which is soluble in sulphuric acid, and notgel per se which would not be soluble in sulphuric acid.

It has now been found that copolymers of terephthalic acid, optionallyincluding isophthalic acid, and mixtures of hexamethylene diamine and2-methyl pentamethylene diamine, may be manufactured without theformation of a high melting fraction in significant amounts.

Accordingly, the present invention provides a process for thepreparation of a partially crystalline copolyamide comprising thepolymerization stages of:

(a) feeding to a reactor an aqueous salt solution of an admixture ofaromatic carboxylic acid and aliphatic diamine, (i) said aromaticcarboxylic acid being selected from of terephthalic acid and mixtures ofterephthalic acid and isophthalic acid and (ii) said aliphatic diaminebeing a mixture of hexamethylene diamine and 2-methyl pentamethylenediamine;

(b) heating the aqueous salt solution under pressure and venting waterand other volatile matter from the reactor;

(c) reducing the pressure in the reactor to atmospheric pressure;

(d) controlling stages (b) and (c) so as to avoid excessive foaming ofthe admixture in the reactor; and

(e) discharging the copolyamide so obtained from the reactor;

the total amount of isophthalic acid, if any, and 2-methylpentamethylene diamine fed to reactor in stage (a) being selected sothat on being annealed the copolyamide has less than 1% by weight, basedon the total amount of copolyamide, of fractions with melting pointsgreater than 320° C.

In preferred embodiments of the process of the invention, the stages ofthe process are controlled so that the copolyamide produced has aninherent viscosity of at least 0.6 dL/g, especially at least 0.8 dL/g.

In a further embodiment of the process of the invention, stage (c)additionally includes maintaining the admixture in the reactor under avacuum.

In another embodiment, the acid and diamine are selected such that thecombined amount of isophthalic acid and 2-methyl pentamethylene diamineis at least 27.5%, molar basis, especially greater than 30%, of thetotal amount of acid and diamine.

The present invention further provides in a process for the preparationof multiple consecutive batches of a partially crystalline copolyamideof terephthalic acid and hexamethylene diamine in a reactor withoutclean-out of the reactor, in which the copolyamide obtained from suchprocess contains more than 1% by weight of a fraction with an annealedmelting point of greater than 320° C., said process comprising thestages of:

(a) feeding to a reactor an aqueous salt solution of an admixture ofaromatic carboxylic acid and aliphatic diamine, (i) said aromaticcarboxylic acid being selected from terephthalic acid and mixtures ofterephthalic acid and isophthalic acid and (ii) said aliphatic diaminebeing a mixture of hexamethylene diamine and 2-methyl pentamethylenediamine;

(b) heating the aqueous salt solution under pressure and venting waterand other volatile matter from the reactor;

(c) reducing the pressure in the reactor to atmospheric pressure;

(d) controlling stages (b) and (c) so as to avoid excessive foaming ofthe admixture in the reactor; and

(e) discharging the copolyamide so obtained from the reactor;

the improvement comprising reducing the amount of the fraction with theannealed melting point of greater than 320° C. to less than 1% byfeeding to the reactor an aqueous solution containing a combined amountof isophthalic acid and 2-methyl pentamethylene diamine of at least27.5%, molar basis, of the total amount of acid and diamine.

In preferred embodiments of the process of the invention, the stages ofthe process are controlled so that the copolyamide produced has aninherent viscosity of at least 0.6 dL/g, especially at least 0.8 dL/g.

In addition, the present invention provides in a process for thecopolymerization of (1) a carboxylic acid consisting of terephthalicacid and mixtures thereof with isophthalic acid with (2) a mixture ofhexamethylene diamine and 2-methyl pentamethylene diamine, theimprovement comprising controlling the combined amount of isophthalicacid and 2-methyl pentamethylene diamine so that the resultantcopolyamide, when annealed, has less than 1% by weight based on thetotal weight of the copolyamide of fractions having melting pointsgreater than 320° C.

As used herein, "clean-out" of a reactor means steps taken to clean orremove high molecular weight polyamide, degradation or gel products andother matter that is not discharged from the reactor under normalprocessing conditions.

The present invention relates to the manufacture of partiallycrystalline copolyamides formed from an aromatic carboxylic acid and amixture of hexamethylene diamine and 2-methyl pentamethylene diamine.The aromatic carboxylic acid is terephthalic acid or a mixture ofterephthalic acid and isophthalic acid.

The amounts of acid and diamines should be substantially complementaryon a molar basis, as will be appreciated by persons skilled in the art.An excess of acids or diamines, especially the latter, may be useddepending on the desired characteristics of the copolyamide and thenature and extent of side reactions that may produce volatile or othermatter; as noted below, diamines tend to be more volatile than acids.

The amount of isophthalic acid plus 2-methyl pentamethylene diamine isselected so that the copolyamide obtained has less than 1% by weight,especially less than 0.1% by weight and in particular less than 0.01% byweight, of fractions having a melting point of greater than 320° C. Inpreferred embodiments, the amount of isophthalic acid plus 2-methylpentamethylene diamine is selected so as to be at least 27.5%, molarbasis, of the total amount of acid and diamine monomers. In morepreferred embodiments, the amount of isophthalic acid plus 2-methylpentamethylene diamine is at least 30%, molar basis, of the total amountof acid and diamine monomers.

The copolyamides produced by the process of the present invention aregenerally partially crystalline polymers, rather than an amorphouspolymer. In embodiments, the polymers have a heat of fusion of greaterthan 17 J/g. Crystallinity may be determined using a differentialscanning calorimeter.

The copolyamide is prepared in a polymerization process in which anaqueous salt solution of the aromatic carboxylic acids, hexamethylenediamine and 2-methyl pentamethylene diamine is fed to a reactor. Thepresence of 2-methyl pentamethylene diamine appears to moderatebranching that is understood to occur in a copolymer of terephthalicacid, isophthalic acid and hexamethylene diamine. The molar amount ofaromatic acid is substantially complementary to the total molar amountof the diamines, as discussed above. As is known, diamines tend to bemore volatile than carboxylic acids and thus it may be desirable to feedan excess of diamine to the reactor. Catalysts may also be used e.g.phosphinic acid and/or the sodium or potassium salts thereof, phosphorusacid, hypophosphorous acid, sodium hypophosphite, phosphoric acid andthe like; typical amounts of catalysts are 0.5-1.00% by weight,especially 0.10-0.20% by weight.

In the process, the aqueous salt solution is heated in a reactor(autoclave) under pressure. The actual pressure used will depend in parton the particular copolyamide that is to be produced in the process. Inpreferred embodiments, the pressure is at least 1300 kPa, preferably atleast 1900 kPa. Water and other volatile matter, which may includediamine, is vented from the reactor. The temperature of the admixture inthe reactor will normally exceed at least 250° C., and especially be inthe range of 270°-310° C. The pressure in the reactor is then reduced toatmospheric pressure, especially over a period of at least 15 minutes,particularly over a period of 20 to 90 minutes. The pressure should bereduced in a manner that minimizes or avoids excessive foaming of thereaction mixture in the reactor. Anti-foam agents may be added to reducethe amount of foaming. The reaction mixture is then maintained at aboutatmospheric pressure or under a vacuum until the copolyamide has reacheda predetermined molecular weight. The copolyamide thus obtained isdischarged from the reactor. It should be understood, however, that thecopolyamides may be manufactured using continuous polymerizationtechniques and/or subjected to solid phase polymerization or otherfurther processing steps.

As is illustrated herein, the use of a polymerization process such asthat described above will often result in the formation of a highmelting fraction of copolyamide, with a melting point after annealing ofgreater than about 320° C., and especially in the range of about330°-335° C. The high melting fraction will normally not be apparent inthe first batch of copolyamide formed in an autoclave i.e. the firstbatch formed after the autoclave has been thoroughly cleaned. The highmelting fraction will normally appear, if it is going to appear, in thesecond or third batches of copolyamide formed in the autoclave. It isnot practical or economic to clean out an autoclave after each and everybatch of copolyamide has been produced.

In order to prevent formation of the high melting fraction, or to atleast reduce the amount of high melting fraction to less than 1% byweight, and especially less than 0.01% by weight, the amount ofisophthalic acid plus 2-methyl pentamethylene diamine in the aqueoussolution fed to the reactor is maintained at at least 27.5% by weight,especially at least 30% by weight.

The copolyamides of the invention may be blended with stabilizers, flameretardants, smoke depressants, plasticizers, conductive and/oranti-static agents, lubricants and mould release agents, nucleatingagents, dyes and pigments, fillers including glass fibres, minerals,toughening and other modifying agents, and other additives that may beused in polyamide compositions. Examples of heat stabilizers includecopper(I) halides e.g. bromide and iodide, and alkali halides e.g.lithium, sodium and potassium bromides and iodides, which may be usedwith or without phosphorus compounds. Examples of the latter arephosphites, phosphines, phosphates and alkali metal salts of phosphorusacids e.g. sodium phenyl phosphinate, sodium hypophosphite, triaryl- andtris(alkylaryl) phosphines e.g. tri-n-butyl phosphine, phenyl dimethylphosphine and triphenyl phosphine. The organic heat stabilizers includehindered phenols and hindered amines, as well as UV stabilizers andphenolic metal deactivators. Nucleating agents include talc, calciumfluoride and salts of phosphorus acids, for example sodium phenylphosphinate.

A wide range of fillers may be used e.g. in amounts of 0.5-200 parts offiller per 100 parts of copolyamide. Examples of such fillers include,but are not limited to, silica, metasilicates, alumina, talc,diatomaceous earth, clay, kaolin, quartz, glass, mica, titanium dioxide,molybdenum disulphide, gypsum, iron oxide, zinc oxide, fibres e.g.glass, carbon, boron, aromatic and ceramic fibres, powderedpolytetrafluoroethylene and the like.

The copolyamides may be used in the manufacture of products using meltprocessing techniques, especially products intended for use attemperatures that are higher than those typically used with otherpolyamides. For example, the copolyamides may be formed into articlesusing injection moulding technology e.g. into valves, tanks, containers,washers and the like for automotive end-uses, into articles forelectrical end-uses e.g. parts requiring resistance to temperatures of260° C. or above, and articles where retention of mechanical propertiesunder the influence of heat, moisture, hydrocarbons, alcohols includingso-called gasohol, and the like are important. Alternatively, thepolymers may be spun into fibres e.g. for sewing or industrial threadsfor end-uses where low shrinkage and elongation are important and/orretention of properties under the influence of moisture, hydrocarbons,alcohols and the like is important. The copolyamides may also be formedinto film and sheet. Barrier properties of the copolyamides to water andoxygen may also find uses. The copolyamides may be particularly usefulfor end-uses where retention of properties at elevated temperatures isrequired, including as retortable containers.

In embodiments of the invention, the copolyamide is in the form of afibre, or filament. The fibre preferably has a tenacity of at least 1.5g/denier and a modulus of at least 30 g/denier.

The present invention is illustrated by the following examples.

EXAMPLE I

A 12 L reaction vessel equipped with a helical ribbon agitator wascharged with 2393.1 g (14.42 moles) of terephthalic acid, 265.9 g (1.60moles) of isophthalic acid, 976 g (8.41 moles) of 2-methylpentamethylene diamine, 1454.1 g of 67.12% (weight basis) of aqueoushexamethylene diamine solution (8.41 moles), 12 g of 47% (weight basis)of aqueous sodium phenylphosphinate solution, 6 ml of 10% (weight basis)Carbowax® 3350 polyethylene glycol in water and 1100 g of demineralizedwater. Thus, isophthalic acid and 2-methyl pentamethylene diamineconstituted 30.5% of the monomers fed to the reactor.

With the agitator rotating at 50 rpm, the mixture was heated to 130° C.,vented to remove entrained oxygen and then heated to 224° C. With thereaction pressure maintained at 1.90 MPa, volatile matter was releasedover a period of 64 minutes, during which period of time the temperatureof the reaction mixture rose to 275° C. The pressure in the reactionmixture was then reduced to atmospheric pressure over a period of 60minutes, the temperature in the reaction mixture rising to 314° C. Therate of agitation was reduced to 5 rpm. The reaction mixture wasmaintained under a vacuum of 40 kPa for 15 minutes and then the polymerobtained was discharged from the reactor and quenched in a water bath.

The polymer obtained had an inherent viscosity (IV) of 0.96 dL/g;inherent viscosity was measured on a 5.0 g/L solution in m-cresol. Thepolymer had a single melting point of 289° C., as measured bydifferential scanning calorimetry (DSC), a T_(g) of 126° C. (DAM, dry asmoulded), a heat deflection temperature (HDT) of 142° C. (1.8 MPa, DAM)and a notched Izod impact strength of "1.54 ft lb/in in SI". The polymerdid not exhibit the presence of a higher melting fraction.

EXAMPLE II

The vessel of Example I was charged with 2659 g (16.01 moles) ofterephthalic acid, 1093 g (9.42 moles) of 2-methyl pentamethylenediamine, 1182.4 g of 58.53% (weight basis) of aqueous hexamethylenediamine solution (7.72 moles), 12 g of 47% (weight basis) of aqueoussodium phenylphosphinate solution, 6 ml of 10% (weight basis) Carbowax®3350 polyethylene glycol in water and 1100 g of demineralized water.Copolyamide was produced using the procedure of Example I. 2-Methylpentamethylene diamine constituted 28.4% of the total monomers fed tothe reactor; the reaction mixture did not contain isophthalic acid.

The copolyamide obtained had an IV of 0.68 dL/g and a melting point of296° C. The polymer did not exhibit a high melt fraction.

EXAMPLE III

Using the procedure of Example I, a series of polymers were preparedusing different ratios of monomers. The polymers were checked using DSCfor the presence or absence of the high melting fraction.

Tests were performed on unfilled polymer and on polymer filled withglass fibre.

The copolyamide compositions were moulded into test specimens using anEngel® or a Boy injection moulding machine. The test samples were Type Itensile bars as specified in ASTM procedure D638, and 1/4" and 1/8"flexural bars as specified in ASTM procedure D790. The specimens weremoulded using a mould temperature of 80° C. and an extruder barreltemperature of 320° C. Measurements were made using the following ASTMprocedures: Flexural Modulus--D790; Tensile Strength--D638; Notched IzodImpact Strength--D256. Melting point and heat of fusion were determinedunder a nitrogen atmosphere using a differential scanning calorimeter(Du Pont 912 DCDSC), at a rate of temperature rise of 10° C. per minute,the temperature at the top of the endothermic curve being deemed to bethe melting point. Glass transition temperature was determined bydifferential mechanical analysis at a rate of increase in temperature of5° C./minute, operated under a nitrogen atmosphere.

Storage Modulus is measured using the procedure of ASTM D 4065-82,Measurements are made of viscoelastic behaviour using dynamic mechanicalanalysis, which provides information on an elastic component, known asstorage modulus, and on a viscous component, known as loss modulus.Measurements made while the temperature is being changed lead toinformation on the T_(g) of the polymer, a heating rate of 5° C. beingused; data on T_(g) reported herein was obtained from a plot of storagemodulus against temperature, being the temperature of significantdecrease in storage modulus with increasing temperature.

Further details and the results obtained are given in Table I.

                  TABLE I                                                         ______________________________________                                        Run No.      1        2        3      4                                       ______________________________________                                        Polymer                                                                       Composition*                                                                  T            50       50       47.5   45                                      I            --       --       2.5    5                                       HMD          25       22.5     25     25                                      MPMD         25       27.5     25     25                                      High Melting yes      no       yes    no                                      Phase        (severe)          (some)                                         Properties -                                                                  Unfilled Polymer                                                              IV (dL/g)    0.90     0.80     0.92   0.96                                    T.sub.m (°C.)                                                                       303      293      297    289                                     Tensile      88.9     --       89.0   90.0                                    Strength (MPa)                                                                Elongation (%)                                                                             --       --       9      47                                      Flexural     3020     --       3025   2725                                    Modulus (MPa)                                                                 Notched Izod (J/m)                                                                         43       --       46     82                                      T.sub.g (°C.)                                                                       125      --       123    126                                     HDT (1.8 MPa, °C.,                                                                  147      --       146    142                                     DAM)                                                                          Properties -                                                                  Filled Polymer                                                                Glass fibre (wt %)                                                                         35       33       35     34                                      Tensile Strength                                                                           196      167      192    207                                     (MPa)                                                                         Flexural Modulus                                                                           9760     9200     9140   9550                                    (MPa)                                                                         Notched Izod (J/m)                                                                         80       101      91     91                                      HDT (1.8 MPa, °C.,                                                                  264      261      253    236                                     DAM)                                                                          ______________________________________                                        Run No.      5      6      7    8    9    10                                  ______________________________________                                        Polymer                                                                       Composition*                                                                  T            30     35     35   47.5 45   35                                  I            20     15     15   2.5  5    15                                  HMD          45     40     45   27.5 27.5 35                                  MPMD         5      10     5    22.5 22.5 15                                  High Melting Phase                                                                         yes    yes    no   yes  yes  no                                  T.sub.m (°C.)                                                                       285    293    300  303  296  298                                 IV (dL/g)    0.74   0.83   0.63 0.84 0.84 0.94                                ______________________________________                                         *T = terephthalic acid                                                        I = isophthalic acid                                                          HMD = hexamethylene diamine                                                   MPMD = 2methyl pentamethylene diamine                                    

This example shows that copolyamides having a combined amount ofisophthalic acid and 2-methyl pentamethylene diamine in excess of 27.5%do not exhibit the presence of high melting fraction in consecutive runsin the autoclave. In some instances, copolyamides having a combinedamount of isophthalic acid and 2-methyl pentamethylene diamine of 27.5%does show the presence of the high melting fraction.

I claim:
 1. A process for the preparation of a partially crystallinecopolyamide comprising the polymerization stages of:(a) feeding to areactor an aqueous salt solution of an admixture of aromatic carboxylicacid and aliphatic diamine, (i) said aromatic carboxylic acid beingselected from terephthalic acid or a mixture of terephthalic acid andisophthalic acid and (ii) said aliphatic diamine being a mixture ofhexamethylene diamine and 2-methyl pentamethylene diamine; (b) heatingaqueous salt solution under pressure and venting water and othervolatile matter from the reactor; (c) reducing the pressure in thereactor to atmospheric pressure; (d) controlling stages (b) and (c) soas to avoid excessive foaming of the admixture in the reactor; and (e)discharging the copolyamides obtained from the reactor;wherein the totalamount of 2-methyl pentamethylene diamine and, when present, isophthalicacid fed to reactor in stage (a) is selected so that the copolyamide hasless than 1% by weight, based on the total amount of copolyamide, offractions with melting points greater than 320° C.
 2. The process ofclaim 1 in which stage (c) additionally includes maintaining theadmixture in the reactor under a vacuum.
 3. The process of claim 1 inwhich the acid and diamine are selected such that the amount ofisophthalic acid and 2-methyl pentamethylene diamine is at least 27.5%,molar basis , of the total amount of acid and diamine.
 4. The process ofclaim 3 in which the acid and diamine are selected such that the amountof isophthalic acid and 2-methyl pentamethylene diamine is at least 30%,molar basis, of the total amount of acid and diamine.